An effective production of fry is essential for the modern aquaculture business which is growing rapidly throughout the world. The newly born fry having small mouths require rotifers, daphnids, and artemia as live foods for certain stage of growth. Therefore, the capacity of live food production at a farm determines the size of the aquaculture business. The live food animals grow on microalgae. The commercial production of microalgae, however, is not easy and costs highly. In fact, more than 50% of the total production cost of most aquaculture farms has been devoted to the microalgae production.
Carbon dioxide is a main raw material for the microalgal production. Since commercially prepared carbon dioxide is expensive, the carbon dioxide contained in the air is usually used to grow microalgae by blowing air into the culture water. The concentration of the atmospheric carbon dioxide is 0.03% which is only one hundredth of the optimal concentration for the microalgal growth. The suboptimal supply of carbon dioxide reduces the microalgal growth rate, and thus a large culture facility is needed to meet the high demand of microalgae. Five to eight tanks of cultured microalgae are known to be needed to feed one tank of the live food animals.
The present inventor has previously proposed an effective and economical way of microalgal production in U.S. Pat. No. 5,040,486 and in French Patent No. 89 16912. By growing microalgae together with fish in high densities in the same tank, fish provide dissolved carbon dioxide, dissolved ammonia, physical agitation and removal of contaminated protozoa beneficial to microalgae, and microalgae provide consumption of excreted compounds to clean the water beneficial to fish. By increasing the fish density, the carbon dioxide concentration in the culture water can be optimized and the resulted high growth rate allows the culture tank ratio of 1:1 for microalgae and live food animals, respectively.
Microalgae, live food animals and fish fry are small and potentially rapid growing organisms. They may be grown in industrial scale in fermentors like bacteria and yeast with a high efficiency. However, their growth rates depend largely on the environmental conditions, particularly the quality of culture water. The water quality of conventional large outdoor ponds and tanks is hard to control.